A vertical-cavity surface-emitting laser (VCSEL) is known as one of surface-emitting lasers. In the vertical-cavity surface-emitting laser (hereinafter, referred to as a “VCSEL”), an active region is arranged between two reflectors to form an optical cavity in the direction perpendicular to a substrate surface. The VCSEL emits light in the direction perpendicular to the substrate surface.
For such a VCSEL, a surface relief structure is known as one of methods for controlling a transverse mode.
FIG. 2A is a cross-sectional view of a surface-emitting laser having a surface relief structure in the related art. FIG. 2A illustrates a VCSEL 1100.
As illustrated in FIG. 2A, the VCSEL 1100 includes a lower multilayer reflector 1104, a lower cladding layer 1106, an active layer 1108, an upper cladding layer 1110, an oxidized confinement layer 1112, and an upper multilayer reflector 1114 stacked on a substrate 1102.
To form the oxidized confinement layer 1112 by oxidation from side walls of the layer, the VCSEL 1100 has a mesa structure configured to expose at least the oxidized confinement layer 1112.
The transverse-mode oscillation of the VCSEL 1100 is dominated by the oxidized confinement layer 1112. For example, a fundamental mode 1130 and a higher-order mode 1132 (here, a first-order mode) having light-intensity distributions as illustrated in FIG. 2A are present.
To allow the VCSEL to oscillate selectively in the transverse mode, for example, a protruding surface relief structure 1120 is formed on a surface of the upper multilayer reflector 1114 of the VCSEL.
The upper multilayer reflector has different reflectances in different regions thereof owing to the surface relief structure. It is thus possible to adjust the mirror loss between the transverse modes.
Hence, oscillation can occur selectively in a specific transverse mode.
To allow the VCSEL to oscillate selectively in the fundamental mode 1130, NPL 1 discloses surface relief structures 1122 and 1124 as illustrated in FIGS. 2B1 and 2B2, each of the surface relief structures 1122 and 1124 having a high reflectance in the middle portion compared with a reflectance in a peripheral portion of a light exit region.
The light intensity of the fundamental mode 1130 is concentrated in the middle of the light exit region compared with the higher-order mode 1132.
Thus, in the foregoing structure, the mirror loss of the higher-order mode is higher than that of the fundamental mode, suppressing the oscillation in the higher-order mode.
In NPL 1, two structures as illustrated in FIGS. 2B1 and 2B2 are disclosed as the surface relief structure 1120.
That is, the surface relief structure 1122 (hereinafter, referred to as a “protruding surface relief structure”) is a structure in which a peripheral portion 1204 are etched to reduce the reflectance. The surface relief structure 1124 (hereinafter, referred to as a “recessed surface relief structure”) is a structure in which a middle portion 1202 is etched to increase the reflectance.
The reason the reflectance of the multilayer reflector is changed by etching is described below. For a multilayer reflector commonly used as a reflector of a VCSEL, the reflectance of the multilayer reflector varies periodically in response to the etch depth when the multilayer reflector is etched. A multilayer reflector includes pairs of high-refractive-index layers 1206 having an optical thickness of λ/4 and low-refractive-index layers 1208 having an optical thickness of λ/4 as illustrate in FIG. 3A1. FIG. 3A2 is a graph showing the relationship between the etch depth d and the reflectance of light incident on an etched portion when the multilayer reflector is etched by a depth d as illustrated in FIG. 3A1.
Here, the number of the pairs of the layers of the multilayer reflector is 34 pairs before etching. The high-refractive-index layers 1206 have a refractive index of 3.43. The low-refractive-index layers 1208 have a refractive index of 3.14.
A medium 300 of the etched portion is air and has a refractive index of 1. The incident light has a wavelength λ of 680 nm.
As is clear from FIG. 3A2, the refractive index when the multilayer reflector is terminated at the low-refractive-index layer 1208 having an optical thickness of λ/4 is lower than that in the case where the high-refractive-index layer 1206 having an optical thickness of λ/4 is further stacked.
Furthermore, the reflectance is lower than that in the case where the low-refractive-index layer 1208 is removed to expose the high-refractive-index layer 1206.
The reflectance is periodically minimized or maximized at etch depths d corresponding to an optical path of λ/2. For the protruding surface relief structure 1122 illustrated in FIG. 2B1, with respect to the upper multilayer reflector terminated at the high-refractive-index layer 1206 having an optical thickness of λ/4, the high-refractive-index layer 1206 located in the peripheral portion 1204 is removed to reduce the reflectance of the peripheral portion 1204.
For the recessed surface relief structure 1124 illustrated in FIG. 2B2, with respect to the upper multilayer reflector terminated at the low-refractive-index layer 1208 having an optical thickness of λ/4, the low-refractive-index layer 1208 located in the middle portion 1202 is removed to increase the reflectance of the middle portion.
By adjusting the size of the etched region in the in-plane direction, a high-reflectance region is arranged in accordance with the high-intensity range of the fundamental mode 1130, and a low-reflectance region is arranged in the peripheral region in accordance with the high-intensity range of the higher-order mode 1132 as illustrated in FIGS. 2B1 and 2B2.
This allows the VCSEL to oscillate selectively only in the fundamental mode 1130.